GB842084A - Improvements in luminescent materials - Google Patents

Abstract

842,084. Luminescent materials. BRITISH THOMSON-HOUSTON CO. Ltd. Nov. 5, 1956 [Nov. 17, 1955], No. 32992/55. Class 39(1). A luminescent material consists of a solid solution of zinc and barium silicates activated with lead, the molecular proportion of zinc silicate to barium silicate lying between 4 : 1 and 1 : 10, and the material when subjected to excitation by short ultra-violet radiation emitting radiation in the range 3000 Š to 3100 Š. The molecular proportions of ZnO: BaO: SiO 2 used in the starting material are preferably such that the constituent silicates in the zincbarium silicate are substantially zinc orthosilicate (Zn2SiO4) and barium disilicate (BaSi2O5). The more efficient phosphors result from proportions lying in the range between 1Zn 2 SiO 4 :2BaSi 2 O 5 and 2Zn 2 SiO 4 : 1BaSi 2 O 5 , the most efficient resulting from proportions near the 1 : 1 ratio. The materials are fired two or three times during preparation, and formation of the phosphor is assisted if a fluoride is present during the first firing and a chloride is added before subsequent firings. Lead fluoride (PbF2) is preferred as the source of both lead and fluorine, the initial amount of PbF2 added lying between 1“% and 5% per total mols of ZnO + BaO (or their equivalents), and being preferably 2¢%. Alternatively, equivalent amounts of PbO (or other Pb compounds that decompose to form PbO) and BaF2 may be used. BaCl2 2H2O is preferred as the source of Cl, but other chlorides, e.g. ZnCl2 may be used. The BaSIsO5 is preferably prepared in precipitate form, but may be prepared from BaCO3 and SiO2. The phosphor is suitable as a coating on the inside of the envelope of a fluorescent low pressure mercury vapour lamp for therapeutic purposes.